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One thing that is certain in computing is that hardware will fail even if the software is written soundly. For this reason, a disaster recovery plan should be implemented for every database server. Even if you are not the administrator of the server, this article will show you how to back up the data you use.
In this article, Albert Lukaszewski, PhD, author of MySQL for Python, we will have covered:
- When to implement one of several kinds of database backup plans
- What methods of backup and disaster recovery MySQL supports
- How to use Python to back up databases
(For more resources on Phython, see here.)
The purpose of the archiving methods covered in this article is to allow you, as the developer, to back up databases that you use for your work without having to rely on the database administrator. As noted later in the article, there are more sophisticated methods for backups than we cover here, but they involve system-administrative tasks that are beyond the remit of any development post and are thus beyond the scope of this article.
Every database needs a backup plan
When archiving a database, one of the critical questions that must be answered is how to take a snapshot backup of the database without having users change the data in the process. If data changes in the midst of the backup, it results in an inconsistent backup and compromises the integrity of the archive. There are two strategic determinants for backing up a database system:
- Offline backups
- Live backups
Which you use depends on the dynamics of the system in question and the import of the data being stored. In this article, we will look at each in turn and the way to implement them.
Offline backups are done by shutting down the server so the records can be archived without the fear of them being changed by the user. It also helps to ensure the server shut down gracefully and that errors were avoided. The problem with using this method on most production systems is that it necessitates a temporary loss of access to the service. For most service providers, such a consequence is anathema to the business model.
The value of this method is that one can be certain that the database has not changed at all while the backup is run. Further, in many cases, the backup is performed faster because the processor is not simultaneously serving data. For this reason, offline backups are usually performed in controlled environments or in situations where disruption is not critical to the user. These include internal databases, where administrators can inform all users about the disruption ahead of time, and small business websites that do not receive a lot of traffic.
Offline backups also have the benefit that the backup is usually held in a single file. This can then be used to copy a database across hosts with relative ease.
Shutting down a server obviously requires system administrator-like authority. So creating an offline backup relies on the system administrator shutting down the server. If your responsibilities include database administration, you will also have sufficient permission to shut down the server.
Live backups occur while the server continues to accept queries from users, while it's still online. It functions by locking down the tables so no new data may be written to them. Users usually do not lose access to the data and the integrity of the archive, for a particular point in time is assured.
Live backups are used by large, data-intensive sites such as Nokia's Ovi services and Google's web services. However, because they do not always require administrator access of the server itself, these tend to suit the backup needs of a development project.
Choosing a backup method
After having determined whether a database can be stopped for the backup, a developer can choose from three methods of archiving:
- Copying the data files (including administrative files such as logs and tablespaces)
- Exporting delimited text files
- Backing up with command-line programs
Which you choose depends on what permissions you have on the server and how you are accessing the data.
MySQL also allows for two other forms of backup: using the binary log and by setting up replication (using the master and slave servers). To be sure, these are the best ways to back up a MySQL database. But, both of these are administrative tasks and require system-administrator authority; they are not typically available to a developer. However, you can read more about them in the MySQL documentation. Use of the binary log for incremental backups is documented at:
Setting up replication is further dealt with at:
Copying the table files
The most direct way to back up database files is to copy from where MySQL stores the database itself. This will naturally vary based on platform. If you are unsure about which directory holds the MySQL database files, you can query MySQL itself to check:
mysql> SHOW VARIABLES LIKE 'datadir';
Alternatively, the following shell command sequence will give you the same information:
$ mysqladmin variables | grep datadir
| datadir | /var/lib/mysql/ |
Note that the location of administrative files, such as binary logs and InnoDB tablespaces are customizable and may not be in the data directory.
If you do not have direct access to the MySQL server, you can also write a simple Python program to get the information:
mydb = MySQLdb.connect('<hostname>',
cursor = mydb.cursor()
runit = cursor.execute("SHOW VARIABLES LIKE 'datadir'")
results = cursor.fetchall()
print "%s: %s" %(cursor.fetchone())
Slight alteration of this program will also allow you to query several servers automatically. Simply change the login details and adapt the output to clarify which data is associated with which results.
Locking and flushing
If you are backing up an offline MyISAM system, you can copy any of the files once the server has been stopped. Before backing up a live system, however, you must lock the tables and flush the log files in order to get a consistent backup at a specific point. These tasks are handled by the LOCK TABLES and FLUSH commands respectively. When you use MySQL and its ancillary programs (such as mysqldump) to perform a backup, these tasks are performed automatically. When copying files directly, you must ensure both are done. How you apply them depends on whether you are backing up an entire database or a single table.
The LOCK TABLES command secures a specified table in a designated way. Tables can be referenced with aliases using AS and can be locked for reading or writing. For our purposes, we need only a read lock to create a backup. The syntax looks like this:
LOCK TABLES <tablename> READ;
This command requires two privileges: LOCK TABLES and SELECT.
It must be noted that LOCK TABLES does not lock all tables in a database but only one. This is useful for performing smaller backups that will not interrupt services or put too severe a strain on the server. However, unless you automate the process, manually locking and unlocking tables as you back up data can be ridiculously inefficient.
The FLUSH command is used to reset MySQL's caches. By re-initiating the cache at the point of backup, we get a clear point of demarcation for the database backup both in the database itself and in the logs. The basic syntax is straightforward, as follows:
FLUSH <the object to be reset>;
Use of FLUSH presupposes the RELOAD privilege for all relevant databases. What we reload depends on the process we are performing. For the purpose of backing up, we will always be flushing tables:
How we "flush" the tables will depend on whether we have already used the LOCK TABLES command to lock the table. If we have already locked a given table, we can call FLUSH for that specific table:
FLUSH TABLES <tablename>;
However, if we want to copy an entire database, we can bypass the LOCK TABLES command by incorporating the same call into FLUSH:
FLUSH TABLES WITH READ LOCK;
This use of FLUSH applies across the database, and all tables will be subject to the read lock. If the account accessing the database does not have sufficient privileges for all databases, an error will be thrown.
Unlocking the tables
Once you have copied the files for a backup, you need to remove the read lock you imposed earlier. This is done by releasing all locks for the current session:
Restoring the data
Restoring copies of the actual storage files is as simple as copying them back into place. This is best done when MySQL has stopped, lest you risk corruption. Similarly, if you have a separate MySQL server and want to transfer a database, you simply need to copy the directory structure from the one server to another. On restarting, MySQL will see the new database and treat it as if it had been created natively. When restoring the original data files, it is critical to ensure the permissions on the files and directories are appropriate and match those of the other MySQL databases.
Delimited backups within MySQL
MySQL allows for exporting of data from the MySQL command line. To do so, we simply direct the output from a SELECT statement to an output file.
Using SELECT INTO OUTFILE to export data
Using sakila, we can save the data from film to a file called film.data as follows:
SELECT * INTO OUTFILE 'film.data' FROM film;
This results in the data being written in a tab-delimited format. The file will be written to the directory in which MySQL stores the sakila data. Therefore, the account under which the SELECT statement is executed must have the FILE privilege for writing the file as well as login access on the server to view it or retrieve it. The OUTFILE option on SELECT can be used to write to any place on the server that MySQL has write permission to use. One simply needs to prepend that directory location to the file name. For example, to write the same file to the /tmp directory on a Unix system, use:
SELECT * INTO OUTFILE '/tmp/film.data' FROM film;
Windows simply requires adjustment of the directory structure accordingly.
Using LOAD DATA INFILE to import data
If you have an output file or similar tab-delimited file and want to load it into MySQL, use the LOAD DATA INFILE command. The basic syntax is:
LOAD DATA INFILE '<filename>' INTO TABLE <tablename>;
For example, to import the film.data file from the /tmp directory into another table called film2, we would issue this command:
LOAD DATA INFILE '/tmp/film.data' INTO TABLE film2;
Note that LOAD DATA INFILE presupposes the creation of the table into which the data is being loaded. In the preceding example, if film2 had not been created, we would receive an error. If you are trying to mirror a table, remember to use the SHOW CREATE TABLE query to save yourself time in formulating the CREATE statement.
This discussion only touches on how to use LOAD DATA INFILE for inputting data created with the OUTFILE option of SELECT. But, the command handles text files with just about any set of delimiters. To read more on how to use it for other file formats, see the MySQL documentation at:
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(For more resources on Phython, see here.)
Archiving from the command line
If you use a MySQL client with a graphical user interface, how you back up will depend on that client. Depending on your platform, MySQL ships with one or both of the following command-line programs used for archiving:
On every MySQL server, you will find the program mysqldump. On Windows, it is usually located in the same directory as the MySQL server executable. On Unix variants, it will be in /usr/bin/.
This program functions like an automated MySQL client. It accepts login credentials from the command line and, based on the options you give it, it will output the script necessary to recreate the database you specify. The basic syntax is:
mysqldump -u <username> -p --database <dbname>
After providing the username and database name, you are prompted for the password. This is because you have not specified it even though you have indicated with the -p flag that you will use a password to log in. This is the more secure way of running mysqldump. Optionally, you can state the password explicitly after the -p flag, but this is not advisable as it then enters into your shell command history as plain text.
Viewing the backup file
Running this command and providing the appropriate password will cause a dump of the specified database. This will almost certainly run off your screen. To save it to a file, use either a greater than sign (>) or the option --result-file= followed by the filename. A dump of the sakila database would read as follows:
mysqldump -u skipper -p --result-file=sql.dump --database sakila
After that process is finished, you could open sql.dump in your favorite text editor to see the following:
-- MySQL dump 10.11
-- Host: localhost Database: sakila
-- Server version 5.0.51a-3ubuntu5.4
/*!40101 SET @OLD_CHARACTER_SET_CLIENT=@@CHARACTER_SET_CLIENT */;
/*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */;
-- Current Database: `sakila`
CREATE DATABASE /*!32312 IF NOT EXISTS*/ `sakila` /*!40100 DEFAULT
CHARACTER SET latin1 */;
-- Table structure for table `actor`
DROP TABLE IF EXISTS `actor`;
SET @saved_cs_client = @@character_set_client;
SET character_set_client = utf8;
CREATE TABLE `actor` (
`actor_id` smallint(5) unsigned NOT NULL auto_increment,
KEY `idx_actor_last_name` (`last_name`)
) ENGINE=InnoDB AUTO_INCREMENT=201 DEFAULT CHARSET=utf8;
So, the dump file contains all of the MySQL commands necessary to create the infrastructure of the database. If you read on, you will soon encounter the following:
-- Dumping data for table `actor`
LOCK TABLES `actor` WRITE;
/*!40000 ALTER TABLE `actor` DISABLE KEYS */;
INSERT INTO `actor` VALUES (1,'PENELOPE','GUINESS','2006-02-15 04:34:3
02-15 04:34:33'),(4,'JENNIFER','DAVIS','2006-02-15 04:34:33'),(5,'JOHN
The necessary INSERT commands are also included. The dump file is a single file backup of the database.
In addition to the options discussed previously, mysqldump supports several other flags. The most commonly used include:
- --all-databases: Dump all tables in all databases
- --compact: Produce more compact output
- --databases: Dump several databases
- --dump-date: Include dump date with the "Dump completed on" comment if --comments is given
- --flush-logs: Flush the MySQL server log files before starting the dump
- --flush-privileges: Emit a FLUSH PRIVILEGES statement after dumping the MySQL database
- --help: Display help message and exit
- --host: Host to connect to (IP address or hostname)
- --ignore-table=db_name.tbl_name: Do not dump the given table
- --lock-all-tables: Lock all tables across all databases
- --lock-tables: Lock all tables before dumping them
- --log-error=file_name: Append warnings and errors to the named file
- --opt: Shorthand for --add-drop-table --add-locks --create-options --disable-keys --extended-insert --lock-tables --quick --set-charset.
- --quick: Retrieve rows for a table from the server a row at a time
- --result-file=file: Direct output to a given file
- --single-transaction: Includes a BEGIN SQL statement before the data from the server
- --skip-triggers: Do not dump triggers
- --tab=path: Produce tab-separated data files
- --tables: Override the --databases or -B option
- --triggers: Dump triggers for each dumped table
- --verbose: Verbose mode
- --version: Display version information and exit
- --where='where_condition': Dump only rows selected by the given WHERE condition
- --xml: Produce XML output
Complete documentation on mysqldump can be found at http://dev.mysql.com/doc/refman/5.5/en/mysqldump.html
Restoring the data
If you use mysqldump to create a standard MySQL backup as discussed above, you can restore the data using the SOURCE command.
mysqldump -u skipper -p --opt sakila > sakila.sql
After the password is entered and the database backed up to sakila.sql, we can login and restore the data:
It is worth noting that the file created by mysqldump does not always create the database. If you want to specify the database in question, use either of the flags --database or --databases. If you are transferring the backup to another server, you will need to create a database for it first. Note, however, that the database used for the backup need not have the same name as the original. It is possible to import a backup of sakila into a database named alikas. Within the database system, the only time database names become critical is in the use of cross-database references such as triggers. Naturally, if you change the name of the database, you will need to change the references used in all of your calling applications, as well.
Triggers are MySQL procedures that initiate an action based on an event in a table. They allow information in one table to be updated, inserted, or deleted based upon data being inserted, updated, or deleted in another table. For the MySQL documentation on triggers, see: http://dev.mysql.com/doc/refman/5.5/en/triggers.html
Then you will need to ensure the greater server environment is mirrored for a functioning (as opposed to simply stored) backup.
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(For more resources on Phython, see here.)
In addition to mysqldump, Unix, Linux, and NetWare servers also support mysqlhotcopy. This program is a Perl script that backs up live databases. Where mysqldump functions like a MySQL client, mysqlhotcopy is a server administration program. It must be run on the same host as the one that runs the database; it cannot be run remotely.
mysqlhotcopy works by copying the salient files for a database to the directory of your choice. Consequently, it will only work if the user who invokes it has read access to those files. This will always be the root administrator of the system but, depending on your setup, it may include others. In the following examples, it is assumed that the one has appropriate permissions to access the files.
The most basic syntax of mysqlhotcopy is:
mysqlhotcopy <database name> <path for backup>
While that is the most basic call of the program, it will almost never work. You must know the username and password of a user that has access to the database you are archiving. Therefore, it is more commonly called like this:
mysqlhotcopy -u skipper -p secret sakila /path/to/a/directory/
Assuming the credentials and access permissions are valid, mysqlhotcopy will then create a directory in which it stores copies of the files used for the database you specified.
In addition to the syntax shown above, the following options are available with mysqlhotcopy:
It should be noted that mysqlhotcopy is still in beta. Therefore, new functionality will be added, and this list is therefore not exhaustive. A convenient way of accessing the options available through your copy of mysqlhotcopy is to use either man or perldoc from a command-line:
Otherwise, you can use:
Either of these commands will give you the manual page for your local version.
Restoring from mysqlhotcopy is a matter of copying the directory that holds the file archive to its appropriate location on your server. This is usually /var/lib/mysql followed by the database name. If in doubt, see how to access this information for your installation of MySQL in the section Copying the Table Files above.
Backing up a database with Python
As we have seen, there are three methods of archiving a MySQL database that a developer can use:
- Copying the MySQL table files directly
- Exporting data to a delimited text file
- Creating a dumpfile
The first and last of these require special permissions on the server. To use Python to manage the backup merely automates the process but still requires you to have access beyond SELECT.
Using MySQLdb, however, we can export the data with only basic privileges. Simply store the results of the SELECT statement into a variable, format it appropriately, and write it to a file.
In this article, we have seen several ways to back up and restore a MySQL database.
We have looked at:
- When it is advisable to backup a running system instead of shutting it down
- The procedure for taking a snapshot of a running MySQL server
- What backup methods are available to a python database developer
- Which privileges are required when using certain archiving methods
- Setting Up Python Development Environment on Mac OS X [Article]
- Objects in Python [Article]
- Python LDAP Applications: Part 1 - Installing and Configuring the Python-LDAP Library and Binding to an LDAP Directory [Article]
- Python Multimedia [Book]
- Scaling your Application Across Nodes with Spring Python's Remoting [Article]
About the Author :
Albert Lukaszewski is a principal consultant for Lukaszewski Consulting Services in southeast Scotland. He has programmed computers for nearly 30 years. Much of his experience is related to text processing, database systems, and natural language processing (NLP). Currently he consults on database applications for companies in the financial and publishing industries.